Abstract
This study is mainly concerned with a 55 km long, east-west oriented strike section through a thick succession of dominantly clastic post-Variscan deposits in the Spanish Pyrenees south of Andorra. A subdivision into three formations can be made: two formations of Stephanian age, composed of volcanic rocks and fluviatile sediments
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respectively, and one of probable Permian age which has been studied in detail and is entirely composed of terrestrial redbeds. These three formations are characterised by large variations in thickness along strike. Thicknesses vary from 0 to 800 m in the volcanics, from 0 to 300 m in the Stephanian fluviatile sediments and from 0 to 1500 m in the Permian redbeds. The geometry of the sedimentary bodies in all three formations suggests deposition in an east-west oriented graben-like depression, a conclusion which is confirmed by detailed sedimentological analysis of the Permian rocks.
The vertical succession of Permian sediments can be subdivided in six major basinfill sequences which unconformably overlie each other. Internally the basinfill sequences are built up of one or more coarsening-upward/fining-upward cycles. The genesis of these cycles is directly related to dip-slip movement along the basin boundary faults, whereas the unconformities between the basinfill sequences indicate that periods of basin subsidence and sedimentation were followed by periods of deformation and erosion. These changes in tectonic regime as reflected in the sedimentary sequences are interpreted to indicate alternating phases of transtension and transpression in a strike-slip zone. Other sedimentological observations substantiate this interpretation. The basin depocentre shifted in space and time, which resulted in the observed large variations in sediment thickness along strike, together with eastward paleocurrent directions along the basin axis.
Transtension was accompanied by outpouring of andesitic lavas and drape folding on a large scale. During transpressional phases normal faults changed character to reverse faults while folding, uplift and subsequent erosion resulted in the formation of intrabasinal unconformities. Alternation of transpressive and transtensive conditions should be the result of movement along non-planar fault surfaces in a major strike-slip zone. A separate structural study of the Variscan basement of the studied basin revealed the existence of a dextral simple-shear deformation generation of probable Stephano-Permian age. Simple shear in the basement on the one hand, and brittle superficial strike-slip faulting involving transtension and transpression on the other, are considered to be expressions of the same regional stress field at different levels in the Earth's crust.
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